Yet More On Decoupling, Part 4: Op amp macromodels: A cautionary tale

[Part 1 looks at the overall supply impedance seen by a component when the decoupling caps, the voltage regulator and the board traces are taken into account. Part 2 looks at how a real-world supply impedance - comprising decoupling capacitors, regulator output impedance, and PCB traces - responds to a small test current step. Part 3 examines how a varying power supply voltage effects the output of a basic op amp circuit.]

Previously on "Yet More..." - OK, that joke is wearing a bit thin. By the end of part 3 we had just got to the point of seeing that when we apply a current excitation to our modelled power supplies, the voltage variations we see on those supplies punch right through to the op-amp output in a quite predictable way. You want to see what that looks like in the time domain, don't you! So do I, but this is the middle third of the story and there has to be an encounter with a scary monster.

Yes, it's time to start using the models of real amplifiers (if that isn't a contradiction!) supplied with LTSpice, and also imported into it from other vendors. It would be disingenuous of me to pretend that I wasn't expecting trouble from these models - and my fears turned out to be justified.

Lies, damn lies and op-amp models
Op-amp vendors don't supply you with the transistor-level circuits of their products; that would give their secrets away to the competition and would demand a great deal from your simulator if you were, for instance, analysing a filter with 12 op amps, each with 70 or more transistors. They provide 'macromodels' which aim to replicate all the essential behaviours of the real device using as few nodes as possible, for faster running. These models are supposed to be much more accurate than the very simple amplifier archetype which I used in part 3 to demonstrate some home truths about amplifiers.

Testing one amplifier after another in the simulator is easy and quick - and initially created great disappointment; the results that came from the simulation were physically impossible. Figure 4.1 reprises results from the made-up amplifier we looked at in part 3. Closed-loop gains of -1 to -1000 are tested, both for the realistic but worst-case in-phase supply modulation (the set of curves asymptotic to 0dB) and the unrealistic (but commonly used by manufacturers) symmetrical modulation (the lower ones). I'm doing both sets for each amplifier to see what this reveals.

Figure 4.2 shows the results using the model of the LT1355, a nice op amp I've used in the past, with extraordinary slew-rate (400V/µs) for its moderate 12MHz GBW. It looks like a plot of some completely different parameter! These numbers are both physically impossible and inconsistent. The OLPSG is down at around -170dB, a figure so small it is likely not due to attributes of real modelled components but to interaction between SPICE conductance defaults. When feedback is applied, the low-frequency supply rejection actually degrades. It then moves in the wrong direction with frequency, improving rather than getting poorer. Something is clearly amiss here. Clearly, this LT1355 model will be no use in my search for what is happening at the output of the amplifier when the supplies are modulated.